Fuel ether production processes conventionally consist in adding an alcohol to a branched olefin. An example thereof is the methyl tertiobutyl ether (MTBE) production process wherein methanol is added to isobutene contained, for example, in an isobutane steam cracking or catalytic cracking or dehydrogenation C4 cut. A similar process allows to produce ethyl tertiobutyl ether (ETBE) from ethanol and isobutene, but also various ethers such as isopropyl tertiobutyl ether (IPTBE) from isopropanol and isobutene, tertio amyl methyl ether (TAME) from methanol and isoamylene, from ex fluid catalytic cracking (FCC) or ex steam cracker C5 cuts, or from the n-pentenes balanced isomerization process Isofive, or ethyl tertioamyl ether (ETAE) from ethanol and isoamylene.
In general terms, industrial processes comprise a reaction section in which the ether is produced in the liquid phase, at low temperature, 50° C. for example, by reaction of an olefinic cut containing at least one iso-olefin with a monoalcohol, in the presence of an ion exchange resin type catalyst, in one or more reactors in series.
The etherification reaction is very selective towards the iso-olefins of the C4 cut, but it is carried out with excess alcohol so as to cause the reaction equilibrium to shift to the production of ether. The composition of the C4 cut varies according to its origin, which may be steam cracking, catalytic cracking, as well as isobutane dehydrogenation or tertiobutanol dehydration. It generally contains less than 50 wt. % iso-olefins, the rest of the feed consisting of a mixture of hydrocarbons inert towards the etherification reaction.
The etherification reaction stage is then followed by a separation stage whose goal is to separate the ether fraction formed and the unreactive or unreacted hydrocarbons for later use, and the excess alcohol. This separation section can consist of a fractionation column allowing to collect the ether at the bottom and the hydrocarbon cut at the top of the column. The non-converted alcohol is recovered, mainly from the hydrocarbon cut, and recycled upstream from the reaction section.
The main reaction between the olefin and the alcohol competes with a parallel reaction of addition of water to the olefin. For example, in the case of ETBE production from isobutene and ethanol, this parallel reaction leads to the formation of tertiobutyl alcohol (TBA). It is therefore advisable to minimize the proportion of water in the etherification reactor feed.
Water has three main origins in the process, corresponding to the residual water contents of the alcohol and of the olefinic feed used, and to the water content of the recycled alcohol. In fact, the latter is generally extracted by subjecting the hydrocarbon fraction obtained at the separation section outlet to a water wash. The recycled alcohol then has a high water content. This recycling can provide up to 50% of the water feeding the reaction section. Patent application FR-A-2,900,924 filed by the assignee provides an alternative to water washing the hydrocarbon cut to recover the alcohol. This invention then brings down the problem of the delivery of water in ether production processes to the feeds of the process: the alcohol and the olefinic feed.
The water content of the olefinic cut is linked with the treatments this cut is subjected to upstream from the etherification process. In fact, the olefinic cut has variable acetonitrile contents according to whether it results from steam cracking, FCC, a dehydrogenation or dehydration operation. This content is less than 20 ppm. The presence of this impurity in the olefinic feed involves a high risk of inhibition of the catalyst used, i.e. ion exchange resins.
An irreversible association is observed between the acetonitrile and the catalysts used in etherification reactors. It is therefore essential to carry out operations of removal of the acetonitrile present prior to sending the olefinic feed to the reaction section.
Generally, removal of this impurity is performed by water wash in a section dedicated to this treatment. The drawback of this solution is then the water saturation of the olefinic feed. The amount of water fed by means of this washing represents at least half the water that goes into the process.
In general, any nitrile molecule that enters the etherification reaction section and has therefore escaped capture by the feed washing plant is trapped by the catalyst resins (which thus lose their acid character and therefore their catalytic activity), either in a first pass, or because these nitrites are recycled through recycling of the alcohol as described in patent U.S. Pat. No. 5,352,848.
The implementation of a process using an extraction liquid other than water for acetonitrile extraction generally allows to divide by at least two the amount of water at the etherification reactor inlet.